An integrated amplifier circuit is typically constructed from a small chip of semiconductor material upon which an array of active/passive components have been constructed and connected together to form a functioning circuit. Integrated amplifier circuits are generally encapsulated in a plastic housing (chip) with signal, power supply, and control leads or pins accessible for connection to external electronic circuitry. Input signals, which are received by the integrated amplifier circuit through input pins, are processed by active and/or passive components in different amplifier stages (e.g., input stage, turn-around stage, and output stage). The output stage of the amplifier is responsible for driving the amplifiers load through the external pins of the integrated circuit.
Amplifier circuits with wide output signal ranges require that the output voltage supplied by the output stage approaches the power supply limits (or rails) under various output loading conditions while maintaining satisfactory performance characteristics. In bipolar junction transistor (BJT) designs, the performance characteristics are influenced by transistor's collector bias current (Ic), collector-to-base voltage operating point (Vbe), temperature, and other factors resulting from the semiconductor manufacturing process. The amplifier circuits loading requirements and output voltage ranges are generally determined by the output transistor's terminal voltages, such as the transistors minimum collector-emitter voltage (Vce-min), base-collector junction voltage (Vbc), and worst case base drive current (Ib). The required quiescent current (IQ) for the transistors in the output stage are typically dictated by the worst case region of operation for the amplifier's performance.